This appears to be Pear leaf Blister Mite. The only treatment I'm afraid is physically removing effected leaves. If I was you, I would prune back the whole tree to three or four year wood to force it to re-shoot. If all leaves left are taken off this should break the cycle of the mite and you will be rid of it for good. It will take a couple of years to grow back flowering and fruiting wood but it will be worth it. There is simply no effective chemical treatment.

Interested to read this as the same thing happened to my plum tree last year. Would this be the same mite do you think. I pruned mine hard last year to reduce it's size by half - I have'nt seen any of these blisters so far this year.

Pear leaf Blister Mite is specific to Pears (Pyrus) so yours must be another problem. Not too sure what the blisters may be on a plum though could just be puckering from aphids?

Thanks for taking the time to answer my question. Your advice has been taken and i will be pruning it back. Thanks again Gisaju